PALEOENVIRONMENTAL CHANGES AT AGOST (SE SPAIN) TRIGGERED BY THE CRETACEOUS/TERTIARY (K/T) BOUNDARY IMPACT

The K/T boundary in the pelagic marls in the Agost section (Betic Cordilleras, SE Spain) is marked by a sharp contact between gray Maastrichtian marls and a 12-cm-thick layer of black clays with a basal 2 mm-thick, ferruginous level enriched in Ir and in Ni-spinels, and containing altered microtektites. Geochemical evidence indicates anoxic/hypoxic conditions during deposition of the black clays. Benthic foraminifera indicate upper bathyal depths during the late Maastrichtian Abathomphalus mayaroensis Biochron through the early Plummerita hantkeninoides Biochron, and middle bathyal depths for the rest of the studied section (latest Maastrichtian through the Parasubbotina pseudobulloides Biochron). Planktic foraminifera suffered a catastrophic mass extinction at the K/T boundary, but benthic foraminifera, in stark contrast, do not show significant extinctions, although faunas underwent a drastic but temporary turnover and a net decrease in infaunal morphogroups. Infaunal groups do not recover to pre-extinction relative abundances, indicating that the food supply to the benthos did not recover fully for at least 200 kyr after the K/T boundary, compatible with the effects of an asteroid impact which destabilized primary producers and the oceanic food-web. In the black boundary clay (low oxygen conditions) foraminifera are rare; the low-diversity faunas varied rapidly and opportunistic taxa show short-lived peaks in relative abundance, reflecting environmental instability and possible delivery of abundant food from local blooms of primary producers. It is not clear whether such blooms could supply sufficient organic matter to sustain the low-oxygen conditions over the several thousands of years reflected in the clays. We speculate that the widespread collapse of the North American continental margin caused by the impact on the Yucatan peninsula might have led to dissociation and oxidation of methane hydrates, accounting for local as well as more widespread low oxygen conditions.